From what I've heard, there are differences between the two, particularly in the way director Mark Romanek is upfront about the tragic scifi story, in which clones-in-love are harvested for organs, in spots where Ishiguro is more withholding. But those differences are surely slight. At the movie's two-thirds point, Kiera Knightley apologizes to her friends, played by Carey Mulligan and Andrew Garfield, for keeping those two lovebirds apart for so long. I was surprised to learn they were even in love. I mean, I got that Mulligan had some sort of crush. But the film's impact hinges upon the epicness of this love, and yet doesn't even take the time to establish it, really. Screenwriter Alex Garland, usually Danny Boyle's collaborator, conspicuously follows Ishiguro's plotting to a fault, to the point that the filmmakers are dutifully moving through a narrative, diagrammatically hitting plot-point touchstones without stopping to consider if anything needs to be developed deeper—if, perhaps, the superficialities don't suffice. As if, there's no time! Ishiguro told a certain story, and its bare shell must be retained at all costs!
Last year, Peter Jackson's adaptation of The Lovely Bones had a similar feel, as did Robert Schwentke's The Time Traveler's Wife. (Both had copious other problems, as well.) Submission to source material has become endemic in our film culture; almost every novel-to-screen adaptation these days suffers from it. The problem results, at least in part, from of our Internet-controlled fanboy culture, in which the devoted nerds can't be counted on to generate buzz unless a film perfectly replicates the thing on which it's based. Even if it's an Alice Sebold novel.
Under this paradigm, filmmakers begin to lose their artist-ness to become reproducers—Xerox machines, producing inferior copies. Film, of course, is a different medium from literature, with its own unique needs and strengths; it functions in its own specific ways that're different from the means by which a book does. But that truth no longer matters. (Except for stage plays; those, like the upcoming Rabbit Hole, are expected to be opened up in adaptation, to unfold in many short scenes across many different settings.) The Watchmen needs to look and move exactly like Alan Moore's comic, otherwise it's inauthentic—never mind if it's vapid and stilted as a result. It's not the substance but the surface that matters. The base might pay an admission price to judge the fealty of the resulting product, but they won't send their friends unless they can award it a passing grade.
It wasn't always like this. Some of American cinema's most fondly remembered films—like, uh, The Wizard of Oz, or Pinocchio—were based on popular books, from which they departed liberally and which they have since long overshadowed. ("Collodi who??") Source material was still permissibly malleable as recently as 20 years ago. I recently caught up with Marek Kanievska's 1987 film adaptation of Less Than Zero; the rights to Bret Easton Ellis' novel had been bought before the book was published, but its content was deemed too risque for audiences. In the ultimate movie, there is exactly one line of dialogue and one moments-long scene lifted from the source material. Everything else is the product of screenwriter imagination. If it makes you wonder why producers would even bother adapting a movie they don't see as adaptable, it's because they're paying for a name: for the buzz that already surrounds a product.
But that buzz is no longer enough. In the Internet culture, buzz must be created and then sustained. In today's landscape, the filmmakers behind an adaptation have to be careful not to offend the diehard fans. Consider the Harry Potter franchise: the last two films—the only ones I saw since Sorcerer's Stone—are briskly plotted: they pare down a dense literary mythology while retaining its emotional core; they move at the swift pace mainstream cinema demands. I haven't read Rowling's novels, either, but I've heard fans complain that too much information has been left out of these movies. For me, that's precisely what makes them successful films in their own rights.
Nevertheless, Harry Potter has proven a valuable film property, so much so that Warner Brothers couldn't just let it go: they split the last book into two parts, to double their profits. This, though, meant more story would be needed to fill the doubled running time. As a result, Harry Potter and the Deathly Hollows: Part 1 does not feel fleetly told as its immediate predecessor: it lumbers along, indulging Rowling's lore to the point that the story ceases to make sense to anyone who hasn't read the book in preparation. And any movie whose narrative coherence depends upon homework done in advance is a narrative failure. It's a(nother) perfect example of a movie that fails by trying to follow its leader too closely.
Movies can never be books, and audiences need to stop insisting that they try. Otherwise, they create the cycle in which we're stuck now, in which the adaptation movie is invariably an obeisant failure. Audiences need to re-allow flexibility to filmmakers, to take characters and themes from sourced stories and recreate them through their own sensibilities. Sure, we might end up with a weird disappointments like Less than Zero. But so what? We could also get another Wizard of Oz.